Method for processing welded metal work joints by high-frequency hummering

ABSTRACT

The inventive method relates to reinforcing treatment of welded metalwork joints using a power ultrasound. Said method can be used for mechanical engineering, shipbuilding, bridge engineering and other branches of industry and construction producing welded structures which can be safely used at conditions of static, dynamic and repeated-variable loading. The inventive method consists in specifying the method for calculating normalised residual compressive stresses to be produced by an ultrasound impact processing. Said stresses are related to the geometrical dimensions of a groove formed when the area disposed along a line between the joint and a base metal is treated. Said invention makes it possible to maximally increase the fatigue limit and the cyclic life duration of welded joints.

[0001] The invention falls into the technological application of high power ultrasonic vibrations for surface hardening treatment of metal products and, first of all, of welded joints of metallic structures with shock impulses of high frequency. It can be utilized in mechanical engineering, shipbuilding, bridge engineering and other branches of industries and construction that are dealing with the manufacturing and maintenance of parts and welded structures working in conditions of dynamic and especially cyclic loading, for prevention of premature fatigue cracks and failures in the zones of stress concentrations and welded joints.

[0002] For hardening and relaxation treatment of welded joints and structures different impact methods are employed: magneto-pulsing, low-frequency hammer peening, shot peening, etc. (G. Danilov et al. Efficiency of technological methods of cyclic life improving of welded elements of sleet proof offshore platforms. Problems of Materials Science. 1996. # 2. p. 15-22). The existing techniques are characterized by considerable power consumption, relatively low efficiency and considerable noise. The high power ultrasonic vibrations are also utilized in fatigue improvement by transforming the ultrasonic energy into the high-frequency impacts of working elements (spheres or rods) that strike the surface of parts or structures with the goal to create plastic deformation of the treated surface (N. Krilov and A. Polishchuk. Application of ultrasonic apparatus for metal texture stabilization. Physical background of industrial application of ultrasound. Part 1. LDNTP. Leningrad. 1970. P. 70-79). The striking force of these elements depends on weight, vibration amplitude of the tip of the ultrasonic horn, radius of the tip of the striking element and on the velocity at the moment of impact.

[0003] The velocity and frequency determine the efficiency of treatment of different materials and welded joints (Patent of Ukraine '12741. Published Feb. 28, 1997). In a known approach the optimum duration of treatment is evaluated by special operating technological complex (Patent of Ukraine # 12741 of Jul. 16, 1993. Method for ultrasonic impact treatment and operational technical system for its realization. B. E. Paton, L. M. Lobanov, E. Sh. Statnikov, E. A. Vostrukhin, S. Zh. Chirtsov, D. E. Aranovsky, V. I. Trufyakov, P. P. Mikheev). For this purpose the alternating electrical voltage in pulsed condition is applied to the magnetostrictive transducer. When voltage is absent the transducer continues to oscillate with some attenuation. When this attenuation is stabilized the treatment is completed. Apparently, the described above technique can be applied to a limited number of materials where mechanical characteristics are essentially changed during treatment. Stronger materials that require considerable duration of treatment, will be handled less than it is required for beneficial effect because the attenuation of oscillations of transducer will be practically identical to initial one at the beginning of treatment. Therefore, for each material and type of welded joint it is necessary to create the optimum technology of treatment that provides the maximum possible effect with minimum power and labor consumption.

[0004] The basic criterion of the increasing of fatigue life of welded elements is the inducing of the normalized on value and character of the distribution of the beneficial compressive residual stresses in the weld zone. These beneficial residual stresses could be induced by impact treatment with the help of high power Ultrasonics.

[0005] The invention falls into the technological application of high power ultrasonic vibrations for surface hardening treatment of metal products and, first of all, of welded joints of metallic structures with shock impulses of high frequency. It can be utilized in mechanical engineering, shipbuilding, bridge engineering and other branches of industries and construction that are dealing with the manufacturing and maintenance of parts and welded structures working in conditions of dynamic and especially cyclic loading, for prevention of premature fatigue cracks and failures in the zones of stress concentrations and welded joints.

[0006] For hardening and relaxation treatment of welded joints and structures different impact methods are employed: magneto-pulsing, low-frequency hammer peening, shot peening, etc. (G. Danilov et al. Efficiency of technological methods of cyclic life improving of welded elements of sleet proof offshore platforms. Problems of Materials Science. 1996. # 2. p. 15-22). The existing techniques are characterized by considerable power consumption, relatively low efficiency and considerable noise. The high power ultrasonic vibrations are also utilized in fatigue improvement by transforming the ultrasonic energy into the high-frequency impacts of working elements (spheres or rods) that strike the surface of parts or structures with the goal to create plastic deformation of the treated surface (N. Krilov and A. Polishchuk. Application of ultrasonic apparatus for metal texture stabilization. Physical background of industrial application of ultrasound. Part 1. LDNTP. Leningrad. 1970. P. 70-79). The striking force of these elements depends on weight, vibration amplitude of the tip of the ultrasonic horn, radius of the tip of the striking element and on the velocity at the moment of impact.

[0007] The velocity and frequency determine the efficiency of treatment of different materials and welded joints (Patent of Ukraine '12741. Published 28.02.97). In a known approach the optimum duration of treatment is evaluated by special operating technological complex (Patent of Ukraine # 12741 of Jul. 16, 1993. Method for ultrasonic impact treatment and operational technical system for its realization. B. E. Paton, L. M. Lobanov, E. Sh. Statnikov, E. A. Vostrukhin, S. Zh. Chirtsov, D. E. Aranovsky, V. L Trufyakov, P. P. Mikheev). For this purpose the alternating electrical voltage in pulsed condition is applied to the magnetostrictive transducer. When voltage is absent the transducer continues to oscillate with some attenuation. When this attenuation is stabilized the treatment is completed. Apparently, the described above technique can be applied to a limited number of materials where mechanical characteristics are essentially changed during treatment. Stronger materials that require considerable duration of treatment, will be handled less than it is required for beneficial effect because the attenuation of oscillations of transducer will be practically identical to initial one at the beginning of treatment. Therefore, for each material and type of welded joint it is necessary to create the optimum technology of treatment that provides the maximum possible effect with minimum power and labor consumption.

[0008] The basic criterion of the increasing of fatigue life of welded elements is the inducing of the normalized on value and character of the distribution of the beneficial compressive residual stresses in the weld zone. These beneficial residual stresses could be induced by impact treatment with the help of high power Ultrasonics.

[0009] Closest to the proposed method is a known method of treatment of welded metallic structures made mainly from steel. This method includes the action by an ultrasonic impact instrument in the weld toe zone with pre-selected vibration amplitude of the transducer's tip. The purpose of the treatment is the increasing of fatigue life of welded metallic structures by inducing normalized on value and character of distribution of residual compressive stresses in the weld zone (Patent of Ukraine '23001. Published Jun. 30, 1998). The vibration amplitude A of ultrasonic horn tip in this case is selected based on empirical relationship: $\begin{matrix} {{2,24} \leq \frac{4\quad \pi \quad A\quad f^{2}m}{\sigma_{Y}R^{2}} \leq {3,36}} & (1) \end{matrix}$

[0010] Where f is the frequency of impact impulses, m is the weight of deforming element, σ_(Y) is the yield strength of the considered material, R is the radius of pin's tip. For welded structures made from low-carbon steel the treatment is performed in the zone restricted by line on the primary recrystallization. In welded structures from alloyed and high-strength steels the treatment is performed in the zone restricted by the line on the low tempering. As an optimum value of induced residual compressive stresses the values of the 1.2-1.5 of the yield strength of material in surface layer with thickness of 0.1-0.2 mm and the total depth of the of the compressive residual stresses equals to 1.0-1.2 mm are accepted. The level of residual stresses is achieved by the using of the parameters of treatment chosen from the relation (1). The value of amplitude A is also evaluated from relation (1). This amplitude can be generated by ultrasonic equipment with different power, but the range of optimum power is not specified in the proposed earlier method.

[0011] The main disadvantage of the above-mentioned method is that the treatment of welded joints of different steels should be made in zones restricted by isothermal curves or low tempering. These parameters should be determined experimentally. At the same time, normalized on value and character of distribution of the compressive residual stresses are assumed by such, which are equal to their maximum achievable values exceeding a yield strength of material in 1.2-1.5 times. However, the conditions of in-service cyclic loading such as stress range, stress ratio and stress concentration caused by joint configuration and other factors that essentially effect the fatigue strength of welded joints and a degree of the influence of residual stresses on the fatigue life are not taken into account. The normalized value of residual compressive stresses, created by ultrasonic impact treatment, in zones of stress concentrators leading to the maximum possible increase of the limit stress range and fatigue life of welded joint should be determined differentially, i.e. depending on above mentioned factors.

[0012] The value of these beneficial compressive residual stresses depends under other equal conditions on the treatment time of considered element or productivity of treatment. But in the known method the amplitude A is determined from the relation (1) where the treatment time is not present. The absence of this parameter does not allow providing the recommendations concerning parameters of treatment in terms of optimum application of welded elements. The above-mentioned disadvantages do not allow the selection of unique technological parameters of treatment for the considered welded element made from different materials.

[0013] The important problem in improvement of ultrasonic impact technology or high frequency peening is the optimization of the technological parameters of treatment by criteria of induced residual compressive stresses in the zones of stress concentrators. Thus it is necessary to determine the values of normalized residual compressive stresses for metals with different strength to ensure the maximum possible increase of limit stress range and fatigue life of welded joints of different types. Besides, it is necessary to create the simplified algorithm for the estimation of the optimum parameters of improvement treatment of welded joint without performing of complicated preliminary experimental investigations.

[0014] The improvement of method of treatment of welded joints of metallic structures made from steels and alloys by high-frequency peening is a subject of present invention. The method includes the action of the ultrasonic impact instrument in zones of stress concentration located along the weld toe zone. The beneficial effect is achieved by relieving of harmful tensile residual stresses and introducing of beneficial compressive residual stresses normalized on value, at which the minimum stresses of loading cycle achieve a yield stress—σ_(Y) of material in the zones of stress concentration. Depending on stress ratio, type of joint and stress concentration factor, mechanical properties of material the value of compressive residual stresses which are necessary for realization of maximum increase of fatigue life essentially varies and can be much less then the—σ_(Y). Besides, the improvement treatment of welded joints made from steels and alloys of different strength is carried out by ultrasonic impact instrument in zones of stress concentrations—weld toe by creating a so called “groove” with the width 2-7 mm and the depth from 0.2 up to 1.0 mm.

[0015] The proposed method allows selecting the parameters of high-frequency peening of welded joints depending on mechanical properties of material, type of welded element and stress concentration factor, parameters of cyclic loading and other factors. For achievement of the maximum increase of the fatigue strength of welded joints there is no need to introduce of compressive residual stresses equal to (1.2-1.5)·σ_(Y). Due to the process of relaxation of residual stresses under the action of external loading the induced compressive residual stresses that provide the maximum possible increase in fatigue strength of welded elements could be significantly lower than the yield strength of material.

[0016] The so called normalized value of residual compressive stresses in the stress concentration zone, depending on the mechanical properties of material, type of welded element and stress concentration factor, parameters of cyclic loading is determined according to the following relationship (V. Trufyakov, P. Mikheev and Y. Kudryavtsev. Fatigue strength of Welded Structures. Residual Stresses and Strengthening Treatments. Harwood Academic Publishers GmbH. 1995. 100 p.): $\begin{matrix} {\sigma_{res}^{ls} = \left\{ {\sigma_{Y} + \frac{2\quad \alpha_{\sigma}{R_{\sigma}\left( {\sigma_{S} + \frac{\sigma_{Y}}{\alpha_{\sigma}}} \right)}}{\left( {1 - R_{\sigma}} \right)\left\lbrack {\frac{\left( {\sigma_{S} - \frac{\sigma_{Y}}{\alpha_{\sigma}}} \right)}{\sigma_{a}^{l}} + 2} \right\rbrack}} \right\}} & (2) \end{matrix}$

[0017] Where σ_(res) ^(ls) is the normalized compressive residual stresses, at which the minimum stresses in the stress concentration zone during the cyclic loading reaches the yield stress of material—σ_(Y); σ_(S) is ultimate strength of material; α_(σ) is a stress concentration factor; R_(σ) is a stress ratio; σ_(α) ^(n) is a limit stress amplitude of welded joint in as-welded condition (with high tensile residual stresses).

[0018] The high-frequency peening is applied to all types of welded elements not to the zones with the width of 3-15 mm as in other known prototypes of present invention, but only to the weld toe zone—the zone with maximum stress concentration. The width of the treated zone is usually 2-5 mm. The width depends mainly on the diameter of striking tools. The zone of treatment—weld toe in as-welded condition is characterized by maximum level of harmful tensile residual stresses and high stress concentration. The improvement treatment of wider zone does not provide any beneficial effect for fatigue life improvement and increases the time of treatment only.

[0019] The parameters of improvement treatment: vibration amplitude of ultrasonic horn, size and number of striking tools, speed of treatment, force of pressing of the instrument to the treated element are selected to provide a necessary level of induced compressive residual stresses. At the same time, the so-called groove after treatment has practically the same geometrical parameters: 2-5 mm in width and depth from 0.2 mm to 1 mm. The geometrical parameters of a groove are correlated with the level of residual stresses. Therefore the evaluation of these geometrical parameters and visual inspection of a groove considerably simplify quality control and estimation of the sufficient number of time for optimum treatment of welded elements.

[0020] The inducing of normalized values of residual compressive stresses σ_(res) ^(ls) provides the maximum possible increase of fatigue strength of welded joints. From the other side, such approach allows essentially decreasing the time of treatment and vibration amplitude of ultrasonic horn. It leads to lowering labor and energy consumption as well as the cost of equipment for high-frequency peening.

[0021]FIG. 1 represents the relationship between the normalized compressive residual stresses σ_(res) ^(ls) and stress ratio of cyclic loading for butt joint made from steels of different grade of strength: low-carbon steels—σ_(Y)˜300 MPa (1), low alloy steels—σ_(Y)˜400 MPa (2), high strength steels—σ_(Y)˜600 MPa (3). FIG. 2 shows the same relationship between σ_(res) ^(ls) and R_(Ò) for steels of different strength grades (1—low-carbon steel, 2—low alloy, 3—high-tensile steel) for a fillet welded joint. The presented data shows the effect of stress ratio on the optimum value of compressive residual stresses that should be induced by high frequency peening in the zones of stress concentrations along the weld toe zone. These optimum residual stresses provide the maximum possible increasing of the limit stress range and fatigue life of welded joints. As can be seen from FIG. 1 and FIG. 2 the levels of optimum residual stresses to be induced by high-frequency peening, can be much lower than the yield stress σ_(y) of considered material and could be as low as 0.5 σ_(Y) of considered material. Only at R_(Ò)=0 the level of these residual stresses is equal to σ_(Y). The data on optimum level of compressive residual stresses for different structural materials, types of welded joints are calculated with the help of computation and are used during the high-frequency peening.

[0022] Realization of the proposed method of high-frequency peening of welded joints of metallic structures is based, first of all, on computation of normalized compressive residual stresses σ_(res) ^(ls), which are required for maximum possible increasing of fatigue life. After computation of σ_(res) ^(ls) it is necessary to select the parameters of improvement treatment by setting the parameters of the ultrasonic generator and the piezoceramic or magneto-strictive transducer. The vibrations of the transducer are transformed to high-frequency impacts of special strikers—pins. Depending on the mechanical properties of the material, the power of ultrasonic equipment is selected in the range 0.25 . . . 1.0 kW, the amplitude of ultrasonic horn—20-35 microns, the diameter of peens—2-7 mm. The power and the amplitude are in direct proportion to the σ_(Y). The diameter of peens must be larger for materials with lower mechanical properties. The basic parameters of improvement treatment are: d—the diameter of pins, n—the quantity of pins, R—the radius of the tip of the pin, Q=T/L—the treatment intensity, where L is the length of the weld and T is the time of treatment, A—the amplitude of ultrasonic oscillations of the horn tip, F_(st)—the force applied to the transducer during the treatment, V—the advancement velocity of the instrument along the weld. Usually, F_(st) is equal to 40-50 N and it is a constant parameter for a specific application.

[0023] The optimization of high frequency peening parameters is carried out on samples of welded elements with the purpose to reach the predetermined value of residual stresses, σ_(res) ^(ls) in the shortest possible time. These residual stresses could be measured by different techniques such as X-ray, ultrasonic or other non-destructive methods. After determination of the optimum treatment time, the geometrical parameters of the treated weld groove are measured. Width and depth of the groove are correlated to the degree of plastic deformation and the residual stresses σ_(res) ^(ls). The high-frequency peening of real welded elements is performed using the parameters that were predetermined during the above-mentioned procedure.

EXAMPLE

[0024] Welded specimens made of steel having medium mechanical properties (for example steel 15{tilde over (OAN)}ÍÀ) are selected for high frequency peening. The optimum value of residual stresses σ_(res) ^(ls) is calculated in accordance with the relationship (2). The parameter σ_(res) ^(ls) for a butt welded joint at symmetric loading cycle and a stress ratio R=−1 is equal to 180 MPa. The parameter σ_(res) ^(ls) could be determined also with the help of curves presented in FIG. 1. Then, the oscillation amplitude of the tip of the transducer is selected as A=25 microns. The pins are selected with the following parameters: the diameter d=3 mm, number of pins, n=4, the radius of the tip, R=3 mm. The advancement rate of the instrument during treatment, V=1 m/minute. The length of the weld to be treated is chosen as L=0.28 m. The improvement treatment along the weld toe is performed in a few passes with the advancement rate V and with intermediate measurements of residual stresses σ_(res) ^(ls). When the measured value of σ_(res) ^(ls) coincides with the predetermined value or exceeds it by 5-10%, the treatment is considered complete and the time of treatment Ò is registered.

[0025] In the presented example Ò=1.12 minutes and the treatment intensity of the considered sample is Q=4 minute/m. Further, the welded elements of the whole structure are treated with the above-mentioned treatment intensity. After the high frequency peening, a weld groove with the width b˜3ìì and depth h˜0.5ìì is produced along the weld toe line. The uniformity and quality of treatment is verified by visual survey. In areas with insufficient width or depth of the groove or visible weld defect, the treatment is repeated, if necessary.

[0026] Samples in as-welded condition and after high-frequency peening were tested in a fatigue testing machine ZDM-10 with the following parameters: stress ratio R_(Ò)=−1, frequency 12 Hz, maximum level of cyclic stresses 0.25·σ_(S) (σ_(S)—ultimate strength of material). The fatigue life of the considered welded element in as-welded condition, when averaged through a number of samples, was 10⁵ cycles of loading and after application of improvement treatment—7·10⁵ cycles. The application of high-frequency peening increased the fatigue life of welded samples seven times.

[0027] The technical and economical efficiency of the proposed method is determined by a significant increase in fatigue life of the welded elements with the simultaneous optimization of process of high-frequency peening of welds due to lowering of the time of treatment and power consumption. 

1. A method of improvement treatment of welded joints of metallic structures by high-frequency peening that uses the effect of application of an ultrasonic impact device with determined oscillation amplitude of the horn tip in the zone of the weld. The proposed method differs from other ones by followings parameters: the treatment is performed in stress concentration zones of welds which are located along the transition line from weld to base metal—weld toe, by inducing the predetermined level of compressive residual stresses σ_(res) ^(ls) providing that the minimum cyclic stresses in the stress concentration zone during the loading reach the yield stress of the material σ_(Y). The required value of σ_(res) ^(ls) is calculated, depending on the stress ratio, the type of welded joint, the stress concentration factor and the mechanical properties of the material, by using the following expression: ${\sigma_{res}^{ls} = \left\{ {\sigma_{Y} + \frac{2\quad \alpha_{\sigma}{R_{\sigma}\left( {\sigma_{S} + \frac{\sigma_{Y}}{\alpha_{\sigma}}} \right)}}{\left( {1 - R_{\sigma}} \right)\left\lbrack {\frac{\left( {\sigma_{S} - \frac{\sigma_{Y}}{\alpha_{\sigma}}} \right)}{\sigma_{a}^{l}} + 2} \right\rbrack}} \right\}},$

where, σ_(res) ^(ls) is the normalized compressive residual stress at which the minimum cyclic stresses from external loading in the stress concentration zone achieve the yield strength of the material; σ_(Y) is the yield strength of the material, σ_(S) is the ultimate strength of the material; α_(σ) is the stress concentration factor; R_(σ) is the stress ratio; σ_(α) ^(n) is the limit stress amplitude of welded joint in as-welded condition with high tensile residual stresses
 2. A method as described in claim 1 that differs by the fact that in welded structures made from steel and alloys of different strength during the high-frequency peening due to plastic deformation of metal the groove of width 2-7 mm and depth 0.2-1 mm is formed, with the geometrical parameters that depend on treatment parameters and are connected with the value of σ_(res) ^(ls) for selected material and type of welded element. 